i built myself one a couple of months ago from spare parts from muh main rig...
Specs of my HTPC:-
AMD Sempron64 2500+
Asus K8N-E Deluxe
Advanced Module 512mb DDR400 x1
Maxtor 20GB PATA HDD
MSI FX5200 GC
Samsung DVD Rom & Samsung CDRW
Powerlogic 500w PSU (Pre-sleeved, sort of UV reactive. changed the stupid white molex to UV reactive ones)
Cheapo RM38 Wireless Mini KB & Mouse (best replacement for remote control

sold in PC3 in LYP)
Linksys Wireless USB Adapter
Edifer R501 MK-II connected to on-board audio
(yeah yeah i know...but trust me...this mobo has excellent on board audio)
Panasonic 21" TV (Waiting for cash for a 29" or mebbe even a 34")
Some brandless 4 or 5 year old beige ATX Casing.
its basically a normal PC....change the graphics card to a better one and it could probably even be a gaming rig

only thing left to do with it is to find a way to record tv/astro direct onto the pc. havent really done any research into that yet.... havent even reconnected muh astro yet